Activin and nodal family members of the TGF-Beta superfamily have been shown to initiate the fonnation of mesoderm in nave, isolated animal cap explants, and thereby mimicking the function of Spemann's organizer. Therefore, in the past, significant efforts have been made to decipher the molecular mechanisms behind activin/nodal signaling, with the result that we learned much about the basic framework of activin/nodal signaling. However, important unanswered questions remain. For instance, we still know very little about the target genes regulated by these factors. Such knowledge is required in order to fully understand the downstream events. Recently, we also came to the realization that, in addition to the canonical TGF- Beta signaling pathway, other, non-canonical pathways mediating activin/nodal signaling may exist. Demonstration of the existence of such a pathway and providing details on how it may function is obviously crucial to discovering how these members of the TGF-Beta superfamily influence germ layer development. The goal of this research project is to provide a better understanding of activin/nodal signaling in order to elucidate not only mechanisms of mesoderm induction but to provide general knowledge of TGF-beta3 signaling as well. Specific aims of this proposal are: Aim 1: To identify activin/nodal target genes and characterize the gene function. Aim 2: To investigate the existence of FoxHi (Fast)-independent activin/nodal signaling. Aim 3: To determine the Smad4-independent activin/nodal signaling